This proposed research addresses specific questions regarding the development of the cochlea and the establishment of the topographic organization of spiral ganglion projections to the cochlear nuclei (CN). Cytochemical labeling techniques (NB and Dil) for neuronal tract tracing combined with electrophysiological recording and extensive quantitative light and electron microscopic analyses will be employed. Our recent studies have shown that projections from basal sectors of the spiral ganglion to each subdivision of the CN undergo significant refinement during early postnatal development. Projections to the anteroventral subdivision (AVCN) are more than 50% broader in neonates than in adults when normalized for CN size, and the descending collaterals of these same neurons projecting to the dorsal (DCN) and posteroventral (PVCN) subdivisions are 30% broader than their adult counterparts. Goals of proposed experiments include: 1) To further investigate the details of timing and sequence of topographic refinement during late gestation and early postnatal development of the projections to each subdivision of the CN. 2) To examine projections from the apical cochlea to determine if maturation of its CN projections parallels the basal-to-apical maturation of the organ of Corti. 3) To complete quantitative studies of the effects of neonatal deafening upon the specificity of CN projections examined in adulthood after a lifetime of severe auditory deprivation. 4) To determine whether CN projections develop normally or if significant reorganization occurs as a consequence of restricted lesions of the spiral ganglion created during early development. 5) To determine whether anatomical remodeling of CN projections underlies the apparent loss of frequency selectivity induced by chronic intracochlear electrical stimulation. 6) To explore the mechanisms which underlie normal development of the auditory nerve projections by examining the nature of spontaneous activity (if any) in spiral ganglion. 7) To characterize selected ultrastructural features of postnatal development of the cat organ of Corti at specific frequency locations. The proposed experiments will increase our understanding of the anatomical framework for information processing mechanisms in the developing auditory system and will address important questions regarding critical periods and plasticity. In addition, studies to be conducted in deafened animals have practical implications regarding application of cochlear implants in children.